[Serializer] Fix serializer tries to denormalize null values on nullable properties

src/Symfony/Component/Serializer/Normalizer/AbstractNormalizer.php

@@ -349,6 +349,12 @@ abstract class AbstractNormalizer extends SerializerAwareNormalizer implements N
                     }
                 } elseif ($allowed && !$ignored && (isset($data[$key]) || array_key_exists($key, $data))) {
                     $parameterData = $data[$key];
+                    if (null === $parameterData && $constructorParameter->allowsNull()) {
+                        $params[] = null;
+                        // Don't run set for a parameter passed to the constructor
+                        unset($data[$key]);
+                        continue;
+                    }
                     try {
                         if (null !== $constructorParameter->getClass()) {
                             if (!$this->serializer instanceof DenormalizerInterface) {

src/Symfony/Component/Serializer/Tests/Fixtures/NullableConstructorArgumentDummy.php

@@ -0,0 +1,32 @@
+<?php
+
+/*
+ * This file is part of the Symfony package.
+ *
+ * (c) Fabien Potencier <fabien@symfony.com>
+ *
+ * For the full copyright and license information, please view the LICENSE
+ * file that was distributed with this source code.
+ */
+
+namespace Symfony\Component\Serializer\Tests\Fixtures;
+
+class NullableConstructorArgumentDummy
+{
+    private $foo;
+
+    public function __construct(?\stdClass $foo)
+    {
+        $this->foo = $foo;
+    }
+
+    public function setFoo($foo)
+    {
+        $this->foo = 'this setter should not be called when using the constructor argument';
+    }
+
+    public function getFoo()
+    {
+        return $this->foo;
+    }
+}

src/Symfony/Component/Serializer/Tests/Normalizer/AbstractNormalizerTest.php

@@ -9,6 +9,7 @@ use Symfony\Component\Serializer\Mapping\Factory\ClassMetadataFactoryInterface;
 use Symfony\Component\Serializer\Normalizer\AbstractNormalizer;
 use Symfony\Component\Serializer\Normalizer\ObjectNormalizer;
 use Symfony\Component\Serializer\Tests\Fixtures\AbstractNormalizerDummy;
+use Symfony\Component\Serializer\Tests\Fixtures\NullableConstructorArgumentDummy;
 use Symfony\Component\Serializer\Tests\Fixtures\ProxyDummy;
 use Symfony\Component\Serializer\Tests\Fixtures\StaticConstructorDummy;
 use Symfony\Component\Serializer\Tests\Fixtures\StaticConstructorNormalizer;
@@ -116,4 +117,15 @@ class AbstractNormalizerTest extends TestCase
         $this->assertEquals('baz', $dummy->quz);
         $this->assertNull($dummy->foo);
     }
+
+    /**
+     * @requires PHP 7.1
+     */
+    public function testObjectWithNullableConstructorArgument()
+    {
+        $normalizer = new ObjectNormalizer();
+        $dummy = $normalizer->denormalize(array('foo' => null), NullableConstructorArgumentDummy::class);
+
+        $this->assertNull($dummy->getFoo());
+    }
 }